Recently, lithium ion secondary batteries were developed as rechargeable secondary batteries, and have been widely used in notebook type personal computers, portable information appliances, digital video camera, digital still camera, mobile telephones and many others.
For charging these lithium ion secondary batteries, various charge control devices have been developed. For example, a device capable of charging plural types of secondary batteries using differing charging methods has been disclosed in Japanese Laid-open Patent Publication No. H11-187587. Japanese Laid-open Patent Publication No. H11-187587 shows a charge control circuit used in both nickel-metal hydride secondary battery and lithium ion secondary battery differing in charging method. In the charging method of lithium ion secondary battery, in the initial stage of charging, the charging current is controlled to be constant, and constant current (CC) charging is done. By this CC charging, when the terminal voltage of the battery reaches a constant voltage, for example, 4.2 V or 4.1 V per cell, the operation is transferred to constant voltage (CV) charging.
Regarding the lithium ion secondary battery, as disclosed in Japanese Laid-open Patent Publication No. 2002-51478, it is known to select the charging condition by an instruction from the operator of the electronic appliance in order to extend the battery life. As shown in this Japanese Laid-open Patent Publication No. 2002-51478, as a characteristic of a lithium ion secondary battery, the filling rate in charging is higher when the charging voltage in CV charging is higher. By one charging operation, the appliance can be used for a long time, but the battery life becomes shorter. When the charging voltage is lower, the battery life is longer, but the filling rate is lower and the appliance can be used for a shorter time by one charging operation.
Japanese Laid-open Patent Publication No. H4-109831 proposes a method of storing the number of times of consecutive charging operations on the basis of information signal of battery remaining capacity from an integrating storage circuit, in order to suppress shortening of life of secondary battery. In order to extend the battery life, it is intended to stop charging when the number of times of consecutive charging operations reaches a specified number of times.
From the standpoint of the manufacturer of electronic appliance, it is desirable to be used for a long time. From the standpoint of user of electronic appliance, a longer battery life is also desired.
It is hence an object of the invention to present a charge control device of a secondary battery so that the appliance can be used for a longer time using a single charging, and the battery life can be extended.
The charge control device of the invention is also provided to address the progress of deterioration of lithium ion secondary battery not only by the number of times of charging, but also by the duration from start of use. Also given that the lithium ion secondary battery deteriorates faster when the battery voltage is higher, it is a further object of the invention to present a charge control device of secondary battery capable of using the appliance for a longer time using a single charging, and also to extend battery life.